Gas lift assembly

ABSTRACT

An apparatus ( 11   a ) for introduction of gas into a wellbore containing production tubing ( 12   a ) is provided. The apparatus consists of a conduit ( 30   a ) for location at least partly within the production tubing ( 12   a ), an inlet ( 32   a ) providing communication between the conduit ( 30   a ) and a wellbore annulus, and an outlet ( 34   a ) providing communication between the conduit and a reservoir ( 16 ). The apparatus therefore allows gas to be introduced into reservoir fluids at a distance from introduction of gas into the production tubing, so increasing the effective depth of gas introduction. A method of gas introduction is also described.

[0001] The present invention relates to an apparatus for use in providing artificial lift to a downhole environment. The invention further relates in an alternative aspect to a method of providing such lift.

[0002] Many oil wells are incapable of sufficiently flowing naturally due to insufficient reservoir pressure; this may be an initial problem with the well, or may be due to a decrease in reservoir pressure such as may be experienced over the producing life of a well. In order to increase the flow rate of such wells, it may be necessary to employ “artificial lift”. Artificial lift may be provided by means of a downhole pump, or by the use of gaslift. Another reason for a fall off in well performance is as a consequence of increase in water cut, where water cut is the percentage of water present in the reservoir fluids.

[0003] A typical well comprises a lined borehole with a length of production tubing disposed therein extending to the reservoir area. The lining material is normally referred to as “casing”. The reservoir is generally sealed from the borehole deep in the well by means of a packer located downstream (that is, towards the surface) of the reservoir, with communication to the surface being possible through the production tubing.

[0004] Gaslift is achieved by introducing gas (typically hydrocarbon-produced gas) at high pressure (typically 1000-3000 psia) into the annulus between the production tubing and the casing. Downstream of the packer is located a valve, normally a one-way valve which allows entry of the gas into the production tubing, which then mixes with fluid produced from the reservoir, reducing the density of the fluid thereby reducing the effective well bore pressure, thereby enabling fluids to flow from the reservoir into the well bore. This then facilitates flow of reservoir fluids to surface.

[0005] The depth of the operating valve by which the gas enters the production tubing is crucial to the effectiveness of the gaslift: as the depth of gas entry increases, so too does the well flowrate. Normally, therefore, the operating valve will be located as deeply as possible; typically immediately downstream of the packer.

[0006] As the well ages, it is often desirable to increase depth of gas injection. In such cases it may be necessary to relocate the packer further upstream. This will however require that the “completion” (ie, the production tubing string and associated components) is removed and reinstalled. This procedure is referred to as a workover, and is generally expensive, requiring production to be shut down for a time. For example, a subsea workover typically may cost millions of UK pounds sterling.

[0007] A further disadvantage may be that in wells where the requirement for gas lift was not envisaged at the time the wells were initially designed, casing collapse strength limitation can often severely limit the maximum allowable lift gas injection depth.

[0008] It is among the objects of embodiments of the present invention to obviate or alleviate these and other disadvantages of conventional gaslift systems.

[0009] According to a first aspect of the present invention, there is provided an apparatus for use in a well having a wellbore containing production tubing, the apparatus comprising conduit means for location at least partly within the production tubing, inlet means for allowing communication between the conduit and a wellbore annulus, and outlet means for allowing communication between the conduit and a reservoir of the well.

[0010] This enables a site of introduction of gas into the production tubing to be separated from a site of introduction of gas into the reservoir fluids, thereby increasing the effective depth of gaslift. The apparatus may therefore be conveniently termed a “Gas Lift Deepening Assembly” (GLDA).

[0011] Preferably a first portion of the conduit is provided within the production tubing while a second portion of the conduit is provided upstream of the production tubing.

[0012] Advantageously the first portion comprises a minor portion of the conduit, while the second portion comprises a major portion of the conduit.

[0013] Preferably the outlet means is is provided upstream of the inlet means.

[0014] Preferably the conduit means comprises tubing. For example, the tubing may be jointed or continuous, such as coiled tubing.

[0015] The outlet means may include a valve; and optionally the inlet means may include a further valve; more preferably the valve(s) are one-way valve(s).

[0016] Preferably the inlet means is adapted to cooperate with a pre-existing aperture in the production tubing. For example, the apparatus may be used with production tubing which has previously been used with conventional gaslift systems, and so is prefitted with an inlet valve in an appropriate portion of the production tubing. Alternatively, an aperture may be formed in the production tubing prior to installing the apparatus. Typically a perforating gun may be run on wireline and detonated to form the aperture.

[0017] Preferably also the apparatus comprises seal means for forming a seal at or around the inlet means, in order to seek to prevent direct communication between the annulus and the production tubing. The seal means may take the form of at least one elastomeric seal, for example.

[0018] Preferably the apparatus further comprises securing means for locating the apparatus at a desired point within the production tubing. Preferably also the securing means is selectively engageable, allowing the apparatus to be retrieved if necessary. For example, the securing means may comprise retractable lock dogs, or such other means as will occur to those of skill in the art.

[0019] According to a second aspect of the present invention there is provided a well having a wellbore containing production tubing and further including an apparatus comprising conduit means for location at least partly within the production tubing, inlet means for allowing communication between the conduit and a wellbore annulus, and outlet means for allowing communication between the conduit and a reservoir of the well.

[0020] Preferably the well also provides a casing.

[0021] Preferably also a seal, such as a packer, is provided between the casing and the production tubing.

[0022] According to a further aspect of the present invention there is provided a method of providing artificial lift to a well having a wellbore containing production tubing, the method comprising the steps of:

[0023] providing a conduit means comprising an inlet means and an outlet means;

[0024] locating the conduit means within the production tubing, such that the inlet means is in communication with a wellbore annulus and the outlet means is in communication with a reservoir of the well; and

[0025] introducing gas into the wellbore annulus, such that gas is transferred via the conduit means to reservoir fluids.

[0026] Preferably the outlet means is located further upstream than the inlet means. Preferably also the wellbore further contains a seal within the annulus, located between the inlet means and the outlet means. For example, the seal may comprise a packer.

[0027] These and other aspects of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which FIGS. 1A and 1B show a conventional gaslift assembly;

[0028]FIG. 2 is a graph showing the relationship between gas injection depth and oil production rate for a typical well;

[0029]FIG. 3A shows one embodiment of an apparatus for use in a wellbore in accordance with an aspect of the present invention;

[0030]FIG. 3B shows a portion of the apparatus of FIG. 3A to an enlarged scale; and

[0031]FIGS. 4A and 4B show an alternative embodiment of an apparatus for use in a wellbore in accordance with an aspect of the present invention.

[0032] Referring first of all to FIGS. 1A and 1B, these show a conventional assembly for providing gaslift to a wellbore. A well 10 contains a length of production tubing 12, and may be lined with casing 14 in a manner which is well known to those of skill in the art. At a lower (upstream) end of the well 10 is for example an oil producing reservoir 16, which is sealed from the wellbore annulus 18 by means of a seal such as a packer 20. Oil under pressure leaves the reservoir 16 and enters the production tubing 12, which transfers the oil to surface.

[0033] In order to provide gaslift, the production tubing 12 is provided with an operating valve 22, a one-way valve providing an inlet from the annulus 18 to the production tubing 12. Liftgas, typically hydrocarbon gas at around 1000-3000 psia (which may be produced gas), is introduced into the annulus 18 by means of entry valves 24 located at or near surface. The gas enters the operating valve 22 and passes into the production tubing 12, where it serves to reduce the density of the produced mixture passing downstream along the production tubing 12, thereby aiding extraction from the well 10.

[0034] The production tube 12 may also be provided with a number of unloading valves 26; these allow the use of lower surface liftgas pressures and may only be active during initial production phase for a number of hours.

[0035] As shown graphically in FIG. 2, the effectiveness of gaslift increases as the depth of the operating valve 22 increases. Thus, in order to increase the effectiveness of the assembly shown in FIG. 1A, the packer 20 must be relocated upstream, as shown in FIG. 1B. Such an operation is complex and costly.

[0036]FIGS. 3A and 3B show an apparatus generally designated 11 a for use in a gaslift method in accordance with one embodiment of the present invention. The components and locations of the wellbore and production tubing are substantially identical with those of FIG. 1A and are identified by like numerals, suffixed ‘a’. Located within the production tubing 12 a, however, is a conduit means in the form of a length of tubing, eg coiled tubing 30 a, with an inlet 32 a at one end, and an outlet operating valve 34 a at the other end. The inlet 32 a communicates with an inlet 33 a on the production tubing which may or may not have a pre-existing operating valve 22 a mounted on the production tubing 12 a in order to introduce liftgas into the tubing 30 a, in use. Further, in use the liftgas then exits the tubing 30 a, via outlet operating valve 34 a, giving a lower effective injection point than is possible with conventional apparatus, and hence increased well flow rate.

[0037] Union of the original inlet 33 a and the inlet 32 a is sealed with elastomeric seals 36 a, which prevent the liftgas entering the production tubing 12 a too far downstream.

[0038] If the tubing 30 a is to be installed in production tubing 12 a which does not already possess a suitable inlet 33 a, then perforations in the production tubing 12 a may be created using shaped charges. A straddle packer assembly as known in the art would be required to effect a seal above and below the perforation holes. This would be located atop the apparatus 11 a.

[0039] The whole apparatus 11 a may be inserted into and retrieved from a well 10 a by means of conventional well intervention technology, such as wireline or coil tubing, so requiring no specialised adaptations for its use.

[0040]FIGS. 4A and 4B show an alternative embodiment of the present invention, which may be used in conjunction with a sliding sleeve assembly 40 b. It is common in oilwell completions to include a sliding sleeve assembly 40 b, normally located immediately above a production packer. Such assemblies 40 b typically contain ports 42 b and a sliding door 43 b complete with seals 44 b which are actuable to open the ports 42 b and provide annulus to production tubing 12 b communication. The sliding sleeve assembly 40 b may also contain machined seal bores 46 b to allow location of plugs, etc, and dog recesses 48 b to accept wireline deployed locks.

[0041] If a sliding sleeve assembly 40 b is present in a well 10 b, this may be used to locate an apparatus 11 b of the present invention. FIG. 4B shows such an apparatus 11 b, which shares many parts with the apparatus 11 a illustrated in FIG. 3; like parts are labelled with like numerals. Lock dogs 50 b are provided on the apparatus 11 b, which serve to secure the apparatus 11 b in place relative to dog recesses 48 b in the sliding sleeve assembly 40 b. Seals 36 b above and below the inlet 32 b and a plug 52 b in the sliding sleeve assembly 40 b prevent gas from entering the production tubing 12 b. A flowpath 54 b is provided to allow communication around the device.

[0042] Normally, the sliding door 43 b is opened to allow communication via ports 42 b and inlet 32 b between the annulus and the conduit 30 b. The apparatus 11 b may then be run into the well by wireline, and the lock dogs 50 b actuated to secure it in place.

[0043] It can be seen from the foregoing that one aspect of the present invention provides an apparatus which may be used in well operations to enable deep gaslift to occur without a need for complex rearrangements or modifications of a well. The apparatus further enables the effective injection depth, and hence gaslift efficiency, to be increased in a relatively straightforward manner. 

1. An apparatus for use in a well having a wellbore containing production tubing, the apparatus comprising conduit means for location at least partly within the production tubing, inlet means for allowing communication between the conduit means and a wellbore annulus, and outlet means for allowing communication between the conduit means and a reservoir of the well.
 2. The apparatus of claim 1, wherein a first portion of said conduit means is provided within the production tubing while a second portion of said conduit means is provided upstream of the production comprises a major portion of said conduit means.
 3. The apparatus of claim 2, wherein said first portion comprises a minor portion of said conduit means, while said second portion comprises a major portion of said conduit means.
 4. The apparatus of any preceding claim, wherein said outlet means is provided upstream of said inlet means.
 5. The apparatus of any preceding claim, wherein said conduit means comprises tubing.
 6. The apparatus of claim 5, wherein said tubing is jointed tubing.
 7. The apparatus of claim 5, wherein said tubing is continuous tubing.
 8. The apparatus of claim 5, wherein said tubing is coiled tubing.
 9. The apparatus of any preceding claim, wherein said outlet means includes a valve.
 10. The apparatus of claim 9, wherein said inlet means includes a further valve.
 11. The apparatus of claim 9 or claim 10, wherein said valve(s) is/are one-way valve(s).
 12. The apparatus of any preceding claim, wherein said inlet means is adapted to cooperate with a pre-existing aperture in the production tubing.
 13. The apparatus of any preceding claim, wherein the apparatus comprises seal means for forming a seal at or around said inlet means, in order to seek to prevent direct communication between the annulus and the production tubing.
 14. The apparatus of claim 13, wherein said seal means takes the forms of at least one elastomeric seal.
 15. The apparatus of any preceding claim, wherein the apparatus further comprises securing means for locating the apparatus at a desired point within the production tubing.
 16. The apparatus of claim 15, wherein said securing means is selectively engageable, allowing the apparatus to be retrieved if necessary.
 17. The apparatus of claim 16, wherein said securing means comprises retractable lock dogs.
 18. A well having a wellbore containing production tubing and further including an apparatus comprising conduit means for location at least partly within the production tubing, inlet means for allowing communication between the conduit means and a wellbore annulus, and outlet means for allowing communication between the conduit means and a reservoir of the well.
 19. The well of claim 18, wherein the well also provides a casing.
 20. The well of claim 18 or claim 19, wherein a seal, such as a packer, is provided between the casing and the production tubing.
 21. A method of providing artificial lift to a well having a wellbore containing production tubing, the method comprising the steps of: providing a conduit means comprising an inlet means and an outlet means; locating the conduit means within the production tubing, and that the inlet means is in communication with a wellbore annulus and the outlet means is in communication with a reservoir of the well; and introducing gas into the wellbore annulus, such that gas is transferred via the conduit means to reservoir fluids.
 22. The method of claim 21, wherein the outlet means is located further upstream than the inlet means.
 23. The method of claim 21 or claim 22, wherein the wellbore further contains a seal within the annulus, located between the inlet means and the outlet means.
 24. The method of claim 23, wherein the seal comprises a packer. 